Sedimentation is a widespread technique utilized in particle-size analysis and reference may be had, for example, to the discussion of the state of the art of such techniques in U.S. Pat. No. 4,775,991, commonly owned with the present application, the prior art and literature cited therein, and the original techniques described for such analysis in that patent.
A precondition for precise results is a perfect dispersion of the particles in the suspension at the beginning of the measurement process and the absence of effects which alter the particle size or the settling velocity of the particles in the liquid phase or a slow movement of the liquid phase while the measurement is going on.
Such effects could include dissolution, i.e. the dissolving of material from the particles into the liquid phase when the latter functions as a solvent, and agglomeration of the particles, i.e. the mechanical, chemical or electrostatic adhesion of discrete particles together to form a larger agglomerated particle. Another effect which disturbs the sedimentation analysis is circulation of the liquid which may be induced by buoyancy effects, such as differences in density due to temperature or an inclined (not vertical) positioning of the cuvette. This movement of the liquid is called thermal circulation.
The dispersion of the particles in the liquid phase before commencement of a measurement is effected as a rule by ultrasonic and mechanical agitation of the suspension. While the dissolution of the particles in the liquid phase, even when the latter is a solvent for the material of the particles, can be suppressed or minimized by a prior saturation of the solvent with the material before commencement of the measurement, the suppression of agglomeration requires special and more complex precautions during the measurement process.
According to German Industrial Standard DIN 66 111, for example, a variety of suspension liquids are prescribed for the various powders and certain dispersion agents (surfactants or dispersants) are likewise mandated or suggested to maintain the particles in discrete or nonagglomerated form in the liquid. The concentration of the particles in the liquid phase is also limited. In practice it has been found that the conditions of this German Industrial Standard cannot always be achieved and thus, for powders not named in the German Industrial Standard, there is a continuing search for dispersing agents which may be effective.
With particle-size measuring devices which operate very rapidly, for example in accordance with the light-scattering principle, the development of agglomeration can be directly followed when it occurs by monitoring the change in the nature of the particles in the suspension in periods of seconds to minutes.
Attached hereto is a FIG. 1 which shows the measured particle size for a suspension after termination of ultrasonic dispersion and without ultrasonic dispersion, respectively, and can be considered to represent the agglomeration with time.
A sedimentation analysis normally requires at least several minutes and may require up to several hours. As a consequence of the agglomeration during the measuring process, the proportion of small particles in the particle total is reduced while the frequency of medium-size particles is increased. The proportion of very large particles does not normally or necessarily change because these tend to sediment out rapidly and before agglomeration has had a substantial effect on the particle-size distribution.
Thermal circulation in general does not affect the larger materials which have a high sedimentation velocity but does affect the small particles which settle very slowly. If they are very small, their settling velocity is less than the circulation velocity of the liquid. Hence with thermal circulation the measured transmission will be less than in the ideal case.
I have discovered that it is possible to quantify or evaluate this change in the character of the particles, namely, dissolution agglomeration or thermal circulation, in a unique manner so as to enable the new procedure to be used for the analysis and evaluation of the agglomeration, dissolution or thermal circulation, or, if desired, to correct sedimentation analysis for the stability of the suspension.